add bundled git-archaeology skill, auto-installed for opencode and claude (#565)

* add bundled git-archaeology skill, auto-installed for opencode and claude

ships a SKILL.md teaching agents the underused git history primitives
(pickaxe -S/-G, -L for function/line ranges, --reverse blame, deleted-file
recovery) so they stop scrolling git log -p when blame comes up empty.

introduces a lightweight bundled-skill path alongside the existing
addSkill (npx skills add) flow used for external skills like agent-browser.
SKILL.md is inlined into dist/cli.mjs via esbuild's text loader and written
to <home>/.agents/skills/<name>/SKILL.md at runtime — no network, no version
drift, no per-run install cost.

* fix: register vitest plugin to load .md as text for bundled-skill tests

* fix: drop vite type import from vitest plugin (vite isn't a direct dep)

* fix: load bundled skills via readFileSync so source mode works

esbuild's text loader only applies to the npm-bundled dist/cli.mjs path. the
preview / oss path runs cli.ts directly with node (PULLFROG_FORCE_LOCAL_CLI=1
in runCli.ts#runLocalCli), where node has no idea how to import .md files —
ERR_UNKNOWN_FILE_EXTENSION crashes the action before any agent starts.

switch to runtime readFileSync that checks both candidate locations:
- source mode: <actionRoot>/skills/<name>/SKILL.md (relative to utils/skills.ts)
- bundled mode: <distDir>/skills/<name>/SKILL.md (esbuild copies the tree)

drops the no-longer-needed esbuild text loader, vitest .md plugin, and
ambient *.md type declaration. wiki/skills.md updated with the why.

* fix: write bundled skills to per-agent dirs so claude actually registers them

utils/skills.ts

@@ -1,10 +1,76 @@
 import { spawnSync } from "node:child_process";
+import { existsSync, mkdirSync, readFileSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
 import { log } from "./cli.ts";
 import { getDevDependencyVersion } from "./version.ts";
 
 const skillsVersion = getDevDependencyVersion("skills");
 
+/**
+ * skills bundled with the action runtime. the SKILL.md files live in
+ * `action/skills/<name>/SKILL.md` and are read at runtime — no esbuild loader,
+ * no codegen. this matters because the preview / oss path runs `cli.ts` from
+ * source (see `runCli.ts#runLocalCli`) where esbuild loaders don't apply.
+ */
+const BUNDLED_SKILL_NAMES = ["git-archaeology"] as const;
+
+/**
+ * resolve the on-disk path of a bundled SKILL.md by checking the two locations
+ * the file may live in:
+ *   - source mode (`runLocalCli`): `<actionRoot>/skills/<name>/SKILL.md`,
+ *     reached as `../skills/...` from `utils/skills.ts`.
+ *   - bundled mode (npx published package): `<distDir>/skills/<name>/SKILL.md`,
+ *     reached as `./skills/...` from `dist/cli.mjs`.
+ *
+ * the bundled-mode copy is produced by an esbuild post-build step in
+ * `esbuild.config.js`.
+ */
+function resolveSkillPath(name: string): string {
+  const here = dirname(fileURLToPath(import.meta.url));
+  const candidates = [
+    join(here, "..", "skills", name, "SKILL.md"),
+    join(here, "skills", name, "SKILL.md"),
+  ];
+  for (const candidate of candidates) {
+    if (existsSync(candidate)) return candidate;
+  }
+  throw new Error(`bundled skill not found: ${name} (looked in ${candidates.join(", ")})`);
+}
+
+/**
+ * each agent has its own auto-scan dir under HOME. we write to all of them so
+ * the same `installBundledSkills` call works regardless of which agent is
+ * running, without coupling skills.ts to agent identity.
+ *
+ * verified empirically (PR #565):
+ *   - OpenCode registers skills from `$HOME/.agents/skills/` and `.opencode/skills/`.
+ *   - Claude Code only registers skills from `$HOME/.claude/skills/` —
+ *     it does NOT scan `.agents/skills/`, so writing only there leaves the
+ *     skill on disk but invisible to Claude's `Skill` tool.
+ */
+const SKILL_TARGET_DIRS = [".opencode/skills", ".claude/skills", ".agents/skills"] as const;
+
+/**
+ * write all bundled skills into the fake HOME so OpenCode / Claude Code discover
+ * them via their auto-scan directories.
+ *
+ * called once per agent run from each agent's `run()`. cheap (small file
+ * writes), no network, idempotent.
+ */
+export function installBundledSkills(params: { home: string }): void {
+  for (const name of BUNDLED_SKILL_NAMES) {
+    const content = readFileSync(resolveSkillPath(name), "utf8");
+    for (const targetDir of SKILL_TARGET_DIRS) {
+      const skillDir = join(params.home, targetDir, name);
+      mkdirSync(skillDir, { recursive: true });
+      writeFileSync(join(skillDir, "SKILL.md"), content);
+    }
+  }
+  log.info(`installed bundled skills: ${BUNDLED_SKILL_NAMES.join(", ")}`);
+}
+
 /**
  * install a skill globally via the `skills` CLI.
  *